The ease with which molten magnesium oxidises generally results in significant losses of metal during molten metal processing. This is particularly so for the overall process of high pressure die casting where there is generally a large amount of returns (eg. rejects, biscuits and runner systems) that need to be recycled. Typically, 40-60% of the weight of a casting requires recycling. The difficulty of recycling without large melt losses typically necessitates recycling in a dedicated facility.
Melt losses, and their consequences, add considerably to the cost of die castings because:                up to 10% of purchased metal is lost to dross and sludge in some operations with the industry average for high pressure die casting being approximately 3-5%;        the effect of melt loss is exacerbated each time metal is melted during recycling;        dross and sludge cannot be readily recycled and therefore removal, transport, treatment and disposal of residues attract significant costs;        of the increased risk of inclusions in the cast part with attendant higher scrap rates;        of downtime of the melting furnace and the diecasting machine, and associated labour, to clean out accumulated sludge;        of reduced furnace capacities due to accumulation of sludge; and        due to its insulating effect, the presence of sludge reduces heat transfer from the heating medium to the molten magnesium, which results in poorer temperature control, extension of heating cycles and decreased crucible life due to increased temperatures at the crucible wall.        
Dross is produced through reaction with air and moisture at the surface of the melt. The production of dross can be reduced by ensuring good seals at crucible lids, selection of an effective cover gas, good cover gas distribution to the melt surface, minimisation of melt surface area and reduction of disturbances to the melt surface.
Sludge mainly contains Fe—Mn—Al intermetallic compounds, oxides that have sunk rather than floated, and entrapped magnesium alloy. Intermetallics form because Fe dissolves from the crucible walls and reacts with Mn and Al in the melt. In this way Fe levels are kept low, but it is important to minimise this reaction otherwise sludge volumes and crucible maintenance increase and further additions of Mn may be necessary.
Intermetallics will also form if the temperature of the liquid falls below the equilibrium level set by the concentrations of Fe and Mn in solution in the liquid pool. This level will initially be set by the composition of the incoming metal, but will change with time in the crucible. Intermittent operation of a melting furnace will also lead to the formation of aluminium-rich compounds in the sludge. This in turn leads to increased dissolution of iron from the crucible.
The rate of dissolution of Fe increases with increasing temperature and the driving force for precipitation of intermetallics increases with decreasing temperature. Thus, if there are significant temperature differences in a melting furnace then large amounts of Fe will dissolve at hot spots on the crucible walls and this will result in the precipitation of intermetallics in cooler areas. Because melting involves the introduction of cold material to a melt, the situation in a melting furnace inherently involves hot and cold spots and so has the potential to generate large amounts of sludge.
An arrangement for melting which minimises the formation of dross and sludge would be of significant benefit to the magnesium industry, and particularly the magnesium die casting industry, because it would increase the efficiencies of melting operations and facilitate more efficient recycling of scrap.